Bone grafts are one of the most common transplanted tissues. Worldwide more than 2.2 million bone-grafting procedures are performed annually to repair bone defects in orthopedics, neurosurgery and dentistry (9, 10, 12, 13). Bone autografts and allografts are conventionally used to replace the bone defects.
Autografts are the gold standard to fill the bone defects, because they provide the fastest incorporation without immunological complications (15, 17). Few mature osteoblasts survive the transplantation however the number of precursor cells that stay alive is adequate. The osteogenic potential is derived from these precursor cells. It is well known in the art that there are severe limitations for the use of autografts these include the increased operative time, limited availability and significant morbidity related to blood loss, wound complications, local sensory loss and, most importantly, chronic pain (14, 16, 18, 19).
Allografts as an alternative approach offer similar characteristics with the exclusion of osteogenic cells (9, 10). There is not a standard rule related to the preparation of bone allografts thus several types of them are disclosed in publications, like fresh, fresh-frozen, freeze-dried or demineralized bone allografts (9, 10). Allografts possess osteoconductive and osteoinductive properties but the latter may not be recognized unless the graft is utilized in either morsellized or demineralized form. Complications associated with allografts include fracture, non-union, immunological complications and infection.
Urist and others (3, 20, 21, 22, 23, 24) demonstrated the effect of demineralized bone matrix or bone morphogenetic protein in bone induction. They also demonstrated that the active components of the demineralized bone matrix are the low molecular weight proteins (LMWPs). Bolander et al. (1, 2) coated the demineralized bone graft with additional low molecular weight proteins. This method could enhance osteoinductive potential of the graft. Nowadays, the demineralized allograft is still regarded as the most appropriate allogenic substitute for replace bone defects because it possesses osteoinductive and osteoconductive ability, however, its mechanical properties are not adequate (22). In addition manufacturing of demineralized bone matrix with constant high osteoinductive property is still a challenge. The mineralized or non demineralized bone allografts, for instance freeze-dried, fresh or fresh-frozen allografts possess good mechanical property but their osteoinductive capability is much poor compared with demineralized bone matrix. Rust et al. (4) demonstrated that mesenchymal stem cells (MSC) could differentiate into osteoblasts on the surface of a mineralized bone allograft, which contains the original proteins of normal bone. They found that MSCs could not differentiate on the surface of heat-treated allografts. This observation shows that particular bone proteins may play a key role in the adherence and commitment of MSCs. Booland et al. (5) proved that cells survive the impaction force on the mineralized allograft, which might be during the clinical use. Lewandrowski et al. (6) coated cortical bone grafts with biopolymers to support the adherence of periosteal derived bone cells on the graft's surface.
New biomaterials combined with osteogenic cells are now being developed as an alternative to bone grafts (11). The biomaterials are created to build up a three-dimensional scaffold to which cells can adhere, proliferate and differentiate into functional osteogenic cells (7, 9). Ore et al. (8) demonstrated that the enhanced differentiation of the human bone marrow derived cells on a 70% carbonated apatite, which has a composition similar to bone minerals.
Present inventors previously found that coating of spongy bone with proteins that are known to help cell adherence had not always helped cell adherence onto the bone. While coating with fibronectin increased the number of attached cells, coating with collagen did not help in increasing the number of attached cells. Moreover none of them supported the proliferation of the attached cells.
An optimal graft should have all the advantages of allo-, and autografts, including osteoinductive ability, good mechanical property and immunological compatibility with the host (10, 11). Furthermore preferably it is easily available and does not cause operative burden to harvest it, like autografts.
Our aim was to develop a reliable and safe coating method that ensures the attachment of stem cells onto the surface of mineralized bone allografts. In other words, the object of the present invention was to develop new methods that provide bone grafts that are compatible with the host and do not cause any immunologic complications while at the same time are easily available.